The purpose of this study is to establish a biological basis for the occurrence of a Druckrey effect in the urethane-induced murine lung adenocarcinoma test system. This system was selected because it serves as a prototype for the behavior of mmchanistically similar but less well documented systems. A single administration of urethane will be used to induce a reasonably stable population of lung adenomas in mice. The excess adenocarcinoma incidence attributable to subsequent chronic urethane exposure (incidence in experimental groups minus control, or spontaneous, incidence) will be used along with adenocarcinoma growth rate data to determine 1) the number of stages in the transformation from adenoma to adenocarcinoma, 2) the magnitude of the Druckrey effect (e.g., the value of n in the equation (dose rate) (time to a mean of one tumor per mouse)n equals a constant, 3) the contribution of adenoma cell proliferation to the Druckrey effect, and 4) the contribution of disproportionate adenocarcinoma growth time to the Druckrey effect.